Mesh/Shading Issue in Blender 2.8
First you have faces that don't stick together, you can fix this with an altM > Merge By Distance, and play with the distance parameter in the last operation box (bottom left of the 3D view).



You'll still have artefacts where the triangles are very stretched, you should better use quads, but if you are lazy you can cut your triangles with additional edge loops (use the knife tool) and it will improve the shading.